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United States Patent |
6,188,914
|
Chheda
|
February 13, 2001
|
Method and apparatus for improving link performance and capacity of a
sectorized CDMA cellular communication network
Abstract
A system and method for improving link performance and network capacity of
a CDMA network is described. In a preferred embodiment, at each sector
site, antennas having a relatively narrow horizontal beamwidth are used as
transmit antennas, while antennas having a relatively wide horizontal
beamwidth are used as receive antennas. Narrow beamwidth antennas are also
used to implement hybrid transmit/receive antennas.
Inventors:
|
Chheda; Ashvin (Dallas, TX)
|
Assignee:
|
Nortel Networks Limited (Montreal, CA)
|
Appl. No.:
|
956296 |
Filed:
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October 22, 1997 |
Current U.S. Class: |
455/562.1; 455/561 |
Intern'l Class: |
H04Q 007/20 |
Field of Search: |
455/562,522,277.1,277.2,449,446,561
|
References Cited
U.S. Patent Documents
5276907 | Jan., 1994 | Median | 455/522.
|
5596333 | Jan., 1997 | Bruckert | 455/522.
|
5615409 | Mar., 1997 | Forssen et al. | 455/562.
|
5684491 | Nov., 1997 | Newman et al. | 455/277.
|
5742911 | Apr., 1998 | Dumbrill et al. | 455/562.
|
5936577 | Aug., 1999 | Shoki et al. | 455/422.
|
5960349 | Sep., 1999 | Chheda et al. | 455/446.
|
Primary Examiner: Le; Thanh Cong
Assistant Examiner: Wyche; Myron K.
Attorney, Agent or Firm: Haynes and Boone, LLP
Claims
What is claimed is:
1. A method of improving link capacity of a sectorized code division
multiple access ("CDMA") network, the method comprising steps of:
(a) installing at at least one sector site of said CDMA network a first
antenna having a first beamwidth; and
(b) installing at said at least one sector site of said CDMA network a
second antenna having a second beamwidth;
wherein said first beamwidth is greater than said second beamwidth; and
wherein said first antenna is a receive-only antenna and said second
antenna is a transmit-only antenna.
2. The method of claim 1 further comprising:
(c) installing at said at least one sector site of said CDMA network a
third antenna having beamwidth equal to said first beamwidth, wherein said
third antenna is a receive-only antenna.
3. The method of claim 1 wherein a sectorization of said CDMA network is
three and wherein said first and second beamwidths are approximately
90.degree. and 60.degree., respectively.
4. The method of claim 1 wherein a sectorization of said CDMA network is
four and wherein said first and second beamwidths are approximately
60.degree. and 40.degree., respectively.
5. The method of claim 1 wherein a sectorization of said CDMA network is
six and wherein said first and second beamwidths are approximately
45.degree. and 30.degree., respectively.
6. The method of claim 1 wherein said step (a) further comprises installing
at each sector site of said CDMA network a first antenna having a
beamwidth equal to said first beamwidth.
7. The method of claim 1 wherein said step (b) further comprises installing
at each sector site of said CDMA network a second antenna having a
beamwidth equal to said second beamwidth.
8. The method of claim 2 wherein said step (c) further comprises installing
at each sector site of said CDMA network a third antenna having a
beamwidth equal to said first beamwidth.
9. A code division multiple access ("CDMA") network having improved link
capacity and performance, the CDMA network comprising:
a plurality of cells each comprising a plurality of sectors, wherein each
of said sectors includes a sector site;
a first antenna installed at each of said sector sites; and
a second antenna installed at each of said sector sites;
wherein at each sector site, a beamwidth of said first antenna of said
sector site is greater than a beamwidth of said second antenna of said
sector site; and
wherein said first antenna is a receive-only antenna and said second
antenna is a transmit-only antenna.
10. The CDMA network of claim 9 further comprising:
a third antenna installed at each of said sector sites;
wherein a beamwidth of each of said third antennas is equal to that of each
of said first antennas and wherein the third antenna is a receive-only
antenna.
11. The CDMA network of claim 9 wherein a sectorization of said CDMA
network is three and wherein said beamwidths of each said first and second
antennas are approximately 90.degree. and 60.degree., respectively.
12. The CDMA network of claim 9 wherein a sectorization of said CDMA
network is four and wherein said beamwidths of each said first and second
antennas are approximately 60.degree. and 40.degree., respectively.
13. The CDMA network of claim 9 wherein a sectorization of said CDMA
network is six and wherein said beamwidths of each first and second
antennas are approximately 45.degree. and 30.degree., respectively.
14. A method of improving link capacity of a sectorized code division
multiple access ("CDMA") network, the method comprising steps of:
(a) installing at at least one sector site of said CDMA network a first
antenna having a first beamwidth; and
(b) installing at said at least one sector site of said CDMA network a
second antenna having a second beamwidth equal to two-thirds of said first
beamwidth; and
wherein said first antenna is a receive-only antenna and said second
antenna is a transmit-only antenna.
15. A code division multiple access ("CDMA") network having improved link
capacity and performance, the CDMA network comprising:
a plurality of cells each comprising a plurality of sectors, wherein each
of said sectors includes a sector site;
a first antenna installed at each of said sector sites; and
a second antenna installed at each of said sector sites;
wherein at each sector site, a beamwidth of said second antenna of said
sector site is equal to two thirds the beamwidth of first second antenna
of said sector site; and
wherein said first antenna is a receive-only antenna and said second
antenna is a transmit-only antenna.
Description
TECHNICAL FIELD
The invention relates generally to sectorized code division multiple access
("CDMA") cellular communication networks and, more particularly, to a
technique for improving link performance and capacity of such a network.
BACKGROUND OF THE INVENTION
In cellular wireless communication networks, or "cellular networks," a
served area is divided into cells. Each cell is further divided into
sectors, except in the case of omni-directional cells, in which the entire
cell comprises a single sector. Each cell is served by at least one base
station located at a cell site typically at the center of the cell. All of
the base stations are connected to a message switching center ("MSC") via
a base station controller ("BSC") and hardware links. A plurality of
mobile units are connected to the MSC by establishing radio links with one
or more nearby base stations.
In communication systems that utilize narrow-band modulations, such as
analog frequency modulation ("FM"), the existence of multiple paths
("multipath") causes severe fading. However, with wideband CDMA
modulation, the different paths may be independently received, thereby
greatly reducing the consequences of the multipath fading. However,
multipath fading cannot be completely eliminated due to the occasional
occurrence of unresolved multipath, i.e., multipath that cannot be
independently processed.
Diversity is the approach most commonly used to mitigate multipath fading.
In a CDMA cellular network, or "CDMA network," three forms of diversity
are used. These include:
time
symbol interleaving, error detection, and correction coding
frequency
signal energy is spread over a large bandwidth
space
1. multiple signal paths from simultaneous links between mobile station and
different sectors (soft handoff)
2. RAKE receivers are used to combat the multipath environment by
separately combining signals arriving with different (resolvable)
propagation delays
3. multiple, typically two, antennas at each cell site, wherein all of the
antennas at a single cell site are designed to the same specifications
One of the most important effects achieved by improving a CDMA network is
the increase in the network's capacity; that is, the number of calls that
can be handled by the network at a given time. It should be noted that the
capacity of a CDMA network is soft, i.e., the capacity of the network can
be increased, but with a corresponding decrease in call quality.
CDMA network capacity takes two forms, which are forward link capacity and
reverse link capacity. In practical CDMA networks, forward link capacity
is the limiting form of capacity. The forward link capacity of a CDMA
system is dependant on handoff and forward link transmit power
requirements between sectors and mobile stations. A higher handoff and
higher transmit power requirement will compromise the CDMA capacity. The
following equation relates the forward link capacity to the average
forward traffic channel gain and average soft handoff percentage for a
CDMA network:
N=(1-(f.sub.pilot +f.sub.page +f.sub.synch))/(f.sub.user.sub..sub.--avg
*hrf* v) (1)
where:
N is the number of users an average sector can support;
f.sub.pilot is the fraction of total sector high power amplifier ("HPA")
power allocated for the pilot channel;
f.sub.page is the fraction of total sector HPA power allocated for the
paging channel;
f.sub.synch is the fraction of total sector HPA power allocated for the
synch channel;
f.sub.user.sub..sub.--avg is the average fraction of total sector HPA
power allocated to a user;
hrf is the handoff reduction factor, a calculated value that takes into
account the required resources due to different types of handoff; and
v is the average voice activity factor.
It should be noted from equation (1) that if the factors hrf and
f.sub.user--avg are reduced, the overall forward link capacity of the
network will be increased.
The reverse link pole (i.e., maximum) capacity may be estimated using the
following equation:
N=(W/R)*(1/(E.sub.b /N.sub.o))*(1/v)*F (2)
where:
N is the number of users per sector;
W is the spread-spectrum bandwidth;
R is the data rate;
E.sub.b /N.sub.o is the ratio of energy per bit (E.sub.b) to the noise
power spectral density (N.sub.o);
v is the average voice duty cycle; and
F is the frequency reuse factor.
Frequency reuse factor is the ratio of the interference from mobile units
within a sector to the total interference from mobiles in all sectors.
Wider antennas result in marginally lower frequency reuse factors.
The capacity of a network is typically increased via sectorization. This is
accomplished by the use of directional antennas. A directional antenna
reduces the interference seen at a base station because it only receives
in the direction of the antenna. In fact, if the antenna had no side-lobes
or back-lobes, which reduce the frequency reuse factor (F), the
out-of-interference would be further reduced, increasing F.
Depending on the purpose of the particular CDMA network considered, cell
site separation may be designed based on "link budget" calculations. The
link budget enables the network planners to separate the cell sites as far
as possible, while maintaining adequate coverage or coverage to a given
grade of service. In such cases, the reverse link budget is used to
determine cell site separation.
In other cases, in particular, when there is a surplus link budget, the
cell sites are positioned in closer proximity to one another. In cases
where the network is designed for capacity, there is a surplus link
budget. In these cases, higher capacity means a greater number of users in
a given area.
In FIG. 1, a typical CDMA network is designated generally by a reference
numeral 10. In a preferred embodiment, the system 10 is comprised of a
plurality of cells, represented in FIG. 1 by cells C1 and C2. Each of the
cells C1, C2, is divided into a plurality of sectors S1, S2, S3 and S4,
S5, S6, respectively, through use of a plurality of directional antennas
(FIG. 2) located at or near a respective base station BS1, BS2. Although
the cells C1, C2, are shown as being divided into three sectors, it will
be recognized by those skilled in the art that cells may be subdivided
into one or more sectors depending on the configuration of the system 10.
As previously noted, each cell C1, C2, comprises a base station B1, B2,
respectively, the primary function of which is to provide over-the-air
radio frequency ("RF") communication with mobile units, such as a mobile
unit 12.
Each base station B1, B2, is further connected via a link to a base station
controller ("BSC") 18, which is connected to a mobile switching center
("MSC") 22. As the elements comprising the system 10, as well as the
configuration thereof, are well known in the art, the details thereof will
not be further described, except as necessary to impart a complete
understanding of the present invention.
As illustrated in FIG. 2, for each sector, there are typically two antennas
200, 202, located at a "sector site" 204 thereof. In some cases, as shown
in FIG. 2, one of the antennas 200 functions as a hybrid transmit/receive
antenna, while the other antenna 202 functions as a receive only antenna.
The antennas 200, 202, are usually are separated far enough apart to
ensure that the individual energy that each antenna captures has faded
independently. Generally, both antennas have been of the same
specifications, including antenna beamwidth, material, size, gain, and
others.
Clearly, increasing the capacity and reliability of a CDMA network is a
constant focus of network planners. Therefore, what is needed is a
technique for improving both link performance (i.e., coverage) and
capacity of a sectorized CDMA network.
SUMMARY OF THE INVENTION
The present invention, accordingly, provides a system and method for
improving both link performance and network capacity of a CDMA network. In
a preferred embodiment, at each sector site, antennas having a relatively
narrow horizontal beamwidth are used as transmit antennas, while antennas
having a relatively wide horizontal beamwidth are used as receive
antennas. Narrow beamwidth antennas are also used to implement hybrid
transmit/receive antennas.
A technical advantage achieved with the invention is that using narrow
beamwidth antennas as transmit antennas results in increased forward link
capacity, which is the limiting form of capacity
A technical advantage achieved with the invention is that using wide
beamwidth antennas as receive antennas results in increased reverse link
coverage. In other words, using a narrow antenna to transmit and to
receive would increase capacity but limit coverage, while using a wide
antenna to transmit and receive would reduce capacity but increase
coverage. Using a narrow antenna to transmit and a wide antenna to receive
would increase both capacity and coverage.
Yet another technical advantage achieved with the invention is that, by
increasing coverage, cell sites can be spaced apart a greater distance,
which results in a decrease in the number of cell sites needed to cover a
given area, assuming that the system is designed for maximum coverage. The
added capacity for the greater coverage is a bonus.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a system block diagram of a CDMA network embodying features of
the present invention.
FIG. 2 illustrates a typical arrangement of antennas at a sector site of
the CDMA network of FIG. 1.
FIG. 3 illustrates one arrangement of antennas at a sector site in
accordance with features of the present invention.
FIG. 4 illustrates an alternative arrangement of antennas at a sector site
in accordance with features of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As described in detail above, FIG. 1 illustrates a CDMA network and FIG. 2
illustrates an arrangement of antennas at a cell site of the CDMA network
of FIG. 1.
Simulations have demonstrated that the hrf and the average forward link
transmit power are reduced as the antenna beam width narrows, resulting in
increased forward link capacity. The reason for the increased forward link
capacity is that the side-lobe interference from adjacent sectors is less
and because there is significantly less softer handoff, which consumes
capacity. This increase has been demonstrated to be on the order of
10-15%. The increase in forward link capacity is shared by an increase in
reverse link capacity, since the frequency reuse factor for the narrower
antenna is higher. Table I below presents the frequency reuse factor for
different antenna beamwidths of a tri-cellular embedded network of CDMA
cells:
TABLE I
Horizontal Antenna Beamwidth (.degree.) Frequency Reuse Factor
60 0.604
70 0.591
80 0.573
90 0.566
100 0.536
Reverse link coverage as a function of cell site location becomes an issue
as the antenna beamwidth is narrowed; that is, there is a higher
probability that the reverse link coverage will result in holes, causing
dropped calls and adversely affecting reverse link quality, unless the
cell sites are brought in closer together. In other words, a narrower
antenna system would be fine so long as the cell sites are sufficiently
close together; however, if for example, an existing network of 90.degree.
beamwidth antennas was converted to 60.degree. beamwidth antennas without
costly but necessary cell site relocation, the aforementioned problems
would surface
Again, simulations have shown that the difference between a 90.degree.
antenna and a 60.degree. antenna in a tri-cellular CDMA network
environment results in a mobile average transmit power that is 50-60%
lower for the wider antenna. This is important, because there is a
reduction in transmitter power requirement to overcome noise and
interference, resulting in a lower reverse link dropped call probability,
longer battery life, and an overall increase in reliability. In other
words, there is a smaller likelihood that the upper limit on the mobile
power requirements will be reached. This reduction also means that the
mobile station has lower incidental costs and allows the lower power units
to operate at larger ranges.
In summary, it can be demonstrated that a narrow beamwidth antenna has
positive effects on forward link capacity, while a wide beamwidth antenna
has positive effects on reverse link coverage.
FIG. 3 illustrates an arrangement of antennas embodying features of the
present invention. In particular, as shown in FIG. 3, by using a narrow
beamwidth (e.g., 60.degree.) antenna as a combined transmit/receive
antenna 300 and a wide beamwidth (e.g., 90.degree.) antenna a second
receive antenna 302, both forward link capacity and reverse link coverage
are improved.
In an alternative arrangement, illustrated in FIG. 4, two separate receive
antennas 400, 402, are implemented using wide beamwidth (e.g., 90.degree.)
antennas, while a separate transmit antenna is implemented using a narrow
beamwidth (e.g. 60.degree.) antenna 404. The embodiment shown in FIG. 4 is
more costly to implement, but has the advantages, including that the
antennas 400, 402, are the of the same type, performance, troubleshooting,
and link budget analysis are easier to perform because the antenna
arrangement is symmetrical.
The difference between the wide and narrow antenna beamwidths, in the cases
illustrated in FIGS. 3 and 4, 30.degree., is impacted by link balancing
issues. In other words, the CDMA network improvement is also dependent on
the forward and reverse links being balanced. For example, referring again
to FIG. 1, if the mobile unit 12 is located somewhere between two sectors,
e.g., sector S5 and sector S6, but the forward link to sector S5 is
superior to the forward link to sector S6, the mobile unit 12 would be
handled by sector S5, unless it is in soft handoff with both sector S5 and
sector S6. If, however, the reverse link to sector S6 is better than the
reverse link to sector S5, and the mobile unit 12 is not in handoff with
sector S6, the reverse link capacity is compromised. This is because the
mobile unit 12 is not reverse link power-controlled by sector S6, which in
this case it should be. The frequency reuse factor will reflect this.
Again, the impact on the reverse link capacity is acceptable as long as it
is above the CDMA network 10 forward link limited capacity.
Simulations have demonstrated that the frequency reuse factor for the
embodiment shown in FIG. 3 is approximately 0.549. This is due to
locations in which the mobile unit is not in handoff (controlled by the
forward link antenna) with a sector that has a superior reverse link.
It is also recognized that next-generation networks may use an array of
antennas at each sector. Applying the teachings of the present invention
to such arrays, it is apparent that narrow beamwidth antennas could be
used for the transmit (forward link) antennas, while wider beamwidth
antennas could be used for the receive (reverse link) antennas.
For networks that are to be with two different antennas, the choice of wide
versus narrow antennas is limited by several factors. First, expected
reduction in the reverse link frequency reuse factor determines the
difference between the wide and narrow antenna, as well as the upper limit
on the wide antenna. Second, the excessive softer handoff impact on the
forward link capacity contributes to the upper limit on the narrow
antenna. Third, the path loss or link budget constrains the lower limit in
narrowing the antennas. In particular, antennas that are too narrow will
undoubtedly cause reverse link holes to open, unless cell sites are placed
closer together. In addition, if the narrow antenna is too narrow, there
will be regions in the sector where the narrow antenna is rendered
effectively useless; therefore, only the wide antenna is of use. The loss
of one antenna increases the reverse link Eb/No requirements, due to loss
of special diversity, which ultimately increase the transmit power
requirements of the mobile unit and may reduce reverse link capacity to
the extent that it is the limiting form of capacity. Finally, a very
narrow antenna, though reducing interference along the side of the sector,
may require the sector to increase its transmit power to users along its
sides, as a thermal noise floor exists that does not depend on multiple
access interference.
Narrow antennas can also be utilized at slightly higher gains, while wide
antennas would have slightly lower gains. This would help the reverse link
performance in terms of reverse link coverage holes, while not unduly
hindering the forward link performance for link balancing issues. It
should also be noted that the gains should not be significantly different.
Another point to note is that higher gain antennas are larger in size,
which is a limitation that needs to be addressed and may cost more money.
in addition, the increase in gain is typically incremental, on the order
of 2-4 dB.
The concept of antenna behavior can be further refined if similar antennas
are used, but the receive antenna has a slower roll-off past the 3 dB
point. In other words, the transmit antenna would have the same horizontal
beamwidth as the transmit antenna, but a significantly slower roll-off.
Consequently, if the CDMA network is designed to meet a link budget, the
link budget should be ideally designed with the narrow antenna in mind.
It should be noted that the teachings of the present invention are
applicable to any level of sectorization with a corresponding adjustment
in the antenna beamwidth. For example, as previously described,
90.degree./60.degree. are the reasonable beamwidth choices for a
sectorization of three (i.e., three sectors per cell), while
60.degree./40.degree. and 45.degree./30.degree. are the reasonable
beamwidth choices for sectorizations of four (i.e., four sectors per cell)
and six (i.e., six sectors per cell), respectively.
The above-described techniques could also be used for cells that are not
equally sectorized. In other words, referring again to FIG. 1, if sector
S1 is dealing with a wider perspective than sectors S2 or S3, sector S1
may have a 80.degree./100.degree. combination of antennas at its sector
site, while sectors S2 and S3 each have a 60.degree./90.degree.
combination of antennas.
Although an illustrative embodiment of the invention has been shown and
described, other modifications, changes, and substitutions are intended in
the foregoing disclosure. Accordingly, it is appropriate that the appended
claims be construed broadly and in a manner consistent with the scope of
the invention.
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